Joining devices are known which comprise two operating units, each having a sealing head for sealing (usually, but not necessarily, of the hot-melt type) bottom and/or top ends of filter bags of various types, such as, for example, traditional single-lobed bags, two-lobed bags, bags of tetrahedral shape, etc.
It should be noted that the joining devices are used on machines comprising stations (operating, preferably, but not necessarily, continuously) which are able to form continuous strips of filter material with a tubular shape feeding along a feed direction (horizontal or vertical) into which, at predetermined intervals, a dose of infusion product is placed.
Some examples of prior art joining devices are illustrated in patent documents JP S63 96003, DE 101 26 202, DE 296 07 449 U1 and EP 2 644 514.
These joining devices join a portion of tubular strip which defines a top of a filter bag being formed and a bottom of a next filter bag. More in detail, the sealing heads of the joining devices cyclically move towards each other until coming into contact with the portion of tubular strip and move away until detaching from the tubular strip, performing seals transversal to the tubular strip arranged at regular intervals. The sealing heads are located on opposite sides and transversally to the tubular strip of filter material being fed.
Each sealing head is connected to a supporting body which, in turn, is constrained to a kinematic unit; the kinematic unit moves the supporting body and the sealing head.
Each kinematic unit comprises two axes of rotary movement (of which at least one is motor driven and synchronised with the movement for feeding the tubular strip of filter material), usually arranged side by side with respect to a horizontal plane or a vertical plane, for each supporting body-sealing head unit.
On each of the axes of rotary movement is keyed an eccentric rotary system consisting of a circular plate having a protruding shaft positioned outside the geometrical centre of the circular plate to form a crank system.
Each supporting member is articulated to the eccentric rotary system in such a way as to rotate about the two axes of rotary movement according to a circular trajectory (clockwise or anti-clockwise) and able to move the joining head with an alternating motion towards and away from the continuous strip of filter material at least between an operating position in contact with the tubular strip of filter material and a non-operating position away from the strip of filter material.
As may be noted in FIG. 1, the circular trajectories Ta and Tb of the two supporting body-sealing head units CT define a point of contact PC (which corresponds to a transversal joining zone of the continuous tubular strip of filter material) at the point of tangency of the trajectories of movement Ta and Tb.
However, the joining devices described above have a disadvantage: the sealing heads remain in contact with the continuous tubular strip of filter material for a very short sealing time.
In effect, the sealing heads are effectively in operational contact with the continuous tubular strip of filter material at a single point of contact PC (corresponding to the point of tangency of the circular trajectories Ta and Tb) for a reduced time and this may determine joins defective or insufficient joins/seals in particular on machines having high production speeds.